I had problems getting the 2.4 kernel to work on an Indy with
a R4000PC (100MHz) processor (very old PROM, too).
The solution I found yesterday is to change an entry in
arch/mips/kernel/cpu-probe.c from CPU_R4000SC to CPU_R4000PC.
Is there a reason why only the SC version is thought to be
there, or is it believed to be compatible?

The SC stands for secondary cache. So one would expect that if
your Indy only has a primary cache (PC) and you are detecting
as an Indy with SC, you will crash since there is no secondary
cache.
-Steve